1. Field of the Invention
The present invention relates to a diesel particulate filter and an exhaust emission control system for purifying particulate matter (PM) contained in exhaust gas of diesel engines.
2. Description of the Related Art
In diesel engines, particulate matter (hereinafter arbitrarily called “PM”) primarily containing carbon particles exists in the exhaust gas (emission). Conventionally, in order to remove this PM, to diesel engines, a filter (diesel particulate filter, hereinafter arbitrarily called “DPF”) is installed to the exhaust mechanism and PM is collected by DPF. In addition, in order to meet lowering of DPF functions, the accumulation rate of PM collected in DPF is estimated from exhaust pressure loss and operating history, and periodical combustion purification (DPF regeneration) was carried out or periodical filter replacement was conducted.
However, in the case of regenerating DPF by combusting PM accumulated on DPF, in order to promote PM combustion, in general, a catalyst is equipped to DPF. This catalyst is equipped to combust PM at still lower temperature, and by this, the regenerative property of DPF is intended to be improved. Various kinds of catalysts as described above have been developed to date, and for example, in Japanese Patent Publication No. HEI6-29542, it is proposed to use a perovskite type complex oxide as catalyst.
Temperature of diesel engine exhaust gas is generally low and it is difficult to raise exhaust gas temperature to the temperature to combust PM accumulated in DPF for regeneration during normal operation. Consequently, exhaust gas temperature is, in general, forcibly raised, for regeneration, by post-injection in which fuel is injected in the engine expansion stroke or exhaust stroke or injecting fuel from an injector mounted to the exhaust pipe.
However, forcibly raising exhaust gas temperature by fuel injection may lower fuel economy or deteriorate DPF (primarily, deterioration of a carrier (filter proper) which supports catalyst) by increasing thermal load to DPF. Therefore, as a DPF carrier, material such as SiC, etc. with outstanding heat resistance has been used. However, material such as SiC, etc. is generally expensive and contributes to increased cost.
In addition, when exhaust gas temperature is forcibly raised by fuel injection, previous-step oxidation catalyst is installed to the previous-step part (upstream part) of DPF in order to effectively heat exhaust gas, or subsequent-step oxidation catalyst is installed to the subsequent-step part (downstream part) of DPF in order to oxidize slip HC, which is uncombusted fuel in exhaust gas that has passed DPF. However, these previous-step and subsequent-step oxidation catalysts contribute to high cost, also.
Therefore, it is desired to definitely combust PM to regenerate DPF even at low ambient temperature as is the case of regular operation (continuous regeneration). However, conventional catalyst as set forth in Japanese Patent Publication No. HEI6-29542 does not provide satisfactory performance and continuous regeneration of DPF has not yet been achieved.
Meanwhile, alkali (alkali metal) is an effective catalyst. However, alkali is highly reactive to Si which is frequently contained in the material for DPF filter proper and in the event that alkali is used for DPF catalyst, alkali may degrade the DPF proper.